Modulation of iono-covalency in Ag–O bond of layer-type oxides Agx[Nix/2IITi1−x/2IV]O2 (x=0.85, 0.70)

Layer-type oxides Agx[Nix/2IITi1−x/2IV]O2 (x=0.85, 0.70) have been prepared by cation-exchange reactions using corresponding Na precursors. For x=0.85, a 3R-delafossite phase was obtained as expected, with Ag+ ions at linear sites between [Ni,Ti]O2 sheets. However, x=0.70 was found to exhibit a six-layered rhombohedral structure, which furnishes the rock-salt layer for Ag+ ions between [Ni,Ti]O2 sheets. This structure change has been considered as a consequence of bond transition of Ag–O from covalent to ionic, due to the enhanced covalency of its competing bond [Ni,Ti]–O. DC polarization and ionic conductivity results confirmed that the interaction of Ag+ ions with oxides at rock-salt layer are highly ionic enough to exhibit a conductivity of 3.6×10−5 S cm−1 at 300 K.